1. Field of the Invention
The present invention relates to the domain of synchronization of items of equipment connected by a packet switching network. The present invention relates more specifically to a system for generation of a synchronization signal by stations connected by such a network, the synchronization signal being able to be used to synchronize remote items of equipment.
2. Description of the Prior Art
Progress in the ability of IP networks to transport all types of signal (data or video) has made it possible to use these networks as the “backbone” architecture for video studios. Of capital importance to this change is therefore having a single infrastructure for the transport of data. Whereas in the past, several media were necessary to transport different signal types between items of equipment, the multiplexing properties offered by the IP layer enable a reduction in the number of media necessary: an IP network that links the different items of equipment.
In the prior art, the synchronisation of items of video equipment (cameras, etc.) in a studio is carried out by the transmission of a synchronisation signal commonly called “Genlock” or “Black burst”. For example, the Genlock signal includes information from the start of the image, repeated every 40 ms, and information from the start of the line repeated every 64 μs. The waveforms of synchronisation signals are a function of the format of the image transmitted on the network. For example, for a high definition image, the synchronisation signal has a tri-level form (−300 mV, 0V, +300 mV).
When a synchronisation signal is routed to different items of equipment to be synchronised by a dedicated coaxial cable, a constant transmission time, without jitter is ensured. From such a signal, all items of equipment are able to reconstruct a timing clock that is specific to its functioning, which guarantees that its functioning is rigorously in phase with all the equipment connected to the same network. For example, two cameras synchronised by a Genlock signal circulating on a dedicated coaxial cable each generate a video with different contents but rigorously in frequency and in phase with one another.
A known disadvantage presented by an IP/Ethernet network is that it introduces a strong jitter in a transmission of signals, and particularly for the transmission of a synchronisation signal. The transmission duration is on average constant but different for each packet transmitted. When such a signal is routed by an IP/Ethernet connection to different items of equipment for synchronising, this jitter results in fluctuations in the length of time required for the information carried by the synchronisation signal to reach the equipment.
In the prior art, for a set of devices, for example cameras, connected to an IP network, devices are known to reconstruct at the level of each camera, a timing clock specific to each camera enabling it to overcome jitter. The underlying principle of these synchronization devices is based on a high attenuation of the synchronisation signal jitter amplitude at the level of reception. In such a way, it can be guaranteed that an image generated by a camera connected to the network is rigorously in phase with all of the images generated by neighbouring cameras connected to the same network.
Examples of such synchronization devices are described in the international PCT application FR2007/050918, they act on program clock reference (PCR) signals that represent very accurate reference clock signals. These digital signals are sent to cameras across a network so that they can locally reconstruct clock signals that are in phase with the reference clock.
One purpose of the present invention is to propose an alternative to these synchronization devices of the prior art to generate on a slave station a synchronization signal and a clock signal in accordance with a master synchronization signal and a master clock signal produced by a master station without requiring a regular transmission of specific digital signals between the master station and the slave station.